The results of this study showed that ONSD in comatose group was 6.30 ± 0.60 mm against 5.10 ± 0.47 mm in normal control group. ONSD / EDT ratio in comatose group was 0.27 ± 0.03 against 0.22 ± 0.02 in normal control group. ONSD and ONSD / ETD ratio were significantly higher in comatose patients with Supratentorial lesions than normal controls, which indirectly confirmed the relationship between ONSD and ICP. There was significant higher ONSD and ONSD / ETD ratio in poor prognosis group than in good prognosis group. However, to predict outcome of comatose patients, the ONSD/ ETD ratio was demonstrated better performance than the ONSD.
Monitoring ICP has important clinical significance for the prognosis and therapeutic schedule of comatose patients. As a noninvasive monitoring method of ICP, ONSD has been used for 20 years and expected to be an effective technique to predict the prognosis of patients. Some researches suggest that ONSD increases with the increase of intracranial pressure, and the increase of ICP has a significant correlation with the poor prognosis of neurological function [7, 10–12]. It was found that ONSD increased in 95% of patients with CT indicated ICH or SAH, and the pathological ONSD was 6.6 ± 0.8 mm, ONSD/ETD ratio was 0.29 ± 0.05 against normative 0.19 ± 0.02. They also found the presence of ONSD greater than a threshold of 5.5 mm was significantly suggestive of elevated ICP and warranted for invasive measurement[5]. Our study showed that ONSD in comatose group was significantly higher than that in normal control group. The results of the current study complement the ONSD data in comatose patients with supratentorial lesions.
Some scholars had made exploration on prognostic value of ONSD in patients with TBI and HIE [3, 7, 13–15]. Shahan Waheed et al. found that in patients with blunt brain injury, ONSD of deathes was higher than that of survivals [7]. Another study found that ONSD was a good predictor of mortality (AUC: 0.805) and the cutoff ≥ 7.3 had a sensitivity of 86.4% and a specificity of 74.6%13. In hypoxic-ischemic encephalopathy, ONSD correlated closely with the neurologic outcome[14, 15]. Other scholars found that ONSD showed potential prognostic value for a poor neurological outcome in patients after hemicraniectomy [16]. At present, some studies found that ONSD / ETD is more valuable than ONSD in predicting intracranial pressure[8, 17]. Therefore, it can be inferred that ONSD / ETD may also be more valuable in predicting prognosis. Some researchers found that the changes in ONSD/ETD ratio compared to the baseline was predictive for late malignant progression in patients with malignant middle cerebral artery infarction [18].In cases with traumatic head injury without hemorrhage, there was an inverse correlation between ONSD/ETD ratio and the GOS [19].Our study found that ONSD/ ETD is more valuable in predicting the prognosis of comatose patients, which may be related to the small variability of ONSD / ETD. The prognostic value of ONSD / ETD needs to be further confirmed
In the process of measuring ONSD, we found that the “mediastinal” window was the best window to observe optic nerve, while most previous studies used “spine” window[5, 6, 9], or "abdomen" window[20], or unspecified window when measuring ONSD. In addition, diverse distance and anatomical marks in the processes of measurement may lead to a great variability of results in studies on ONSD. In this case, due to individual differences, monitoring ONSD may be of great value to determine abnormality of ICP and predict prognosis. Therefore, it is very important to establish an unified method to measure ONSD and the range of normal ONSD values .
Whether ONSD is related to age, height, weight, BMI, MBP and other factors has not been determined. Many studies have shown that ONSD has no correlation with the above factors [7, 21, 22]. Like other nerves in the human body, optic nerve fibers are lost with age. However, the average diameter of axons increased with age [23]. At the same time, dural thickness increased with age [24]. Therefore, some scholars believe that ONSD is approximately constant during a lifetime9. Our study showed that these factors were not significantly different between comatose patients and normal controls except MBP, which is related to hemodynamic instability of coma group.
There are some limitations in our study. First, this is a retrospective analysis with a small sample size, which may lead to inevitable bias. Second, only a few of our cases had invasive intracranial pressure monitoring, so we cannot directly link ONSD with ICP. Third, the comatose patients in our study had different etiologies, which may also cause some bias. Multi-center studies with larger sample are needed to further evaluate ONSD in comatose patients with a specific etiology and to determine the relationship of ONSD and ONSD/ETD ratio with prognosis of comatose.
In conclusion, our study showed that ONSD was significantly increased in comatose patients with supratentorial lesions, which indicating an increase of ICP. The ONSD in comatose patients with poor prognosis is higher than those with good prognosis, which means that ONSD measurement could be used as a new method to evaluate the prognosis of comatose patients with supratentorial lesions. Well-designed study with larger sample is needed to further clarify.